In the art of drilling and producing hydrocarbons from an oil well, many various tubular goods, such as drill pipe, production tubing, and sucker rod must be used downhole in the borehole. In producing the well, for example, it is customary to employ a pump-jack unit which reciprocates a string of sucker rod. The sucker rod extends downhole to a downhole pump so that the pump-jack unit reciprocates the rod string which in turn reciprocates the downhole pump and produces oil.
A string of sucker rod is made up of individual joints which are approximately 25 feet in length. The rod joints are connected together by a coupling member. The string of sucker rod may be more than a mile in length. The sucker rod is quite costly and it is advantageous to protect the rod against corrosion.
From time to time, it is necessary to pull the entire rod string out of the borehole so that various different repairs or changes can be made to the oil well. This necessitates unscrewing each joint as the string is lifted from the hole. The sucker rod joints are then stacked until the work on the well has been completed and then the rod joints are made back into a string by reversing the above procedure.
The sucker rods are sometimes coated with foreign matter, including corrosive well fluids, scale, paraffin, oxidation products, and other debris. When the rods are subsequently replaced into the borehole, they continue to deteriorate because the rod is located in the corrosive well fluids.
Corrosion attack on sucker rod steel accounts for about one-half of all sucker rod failures and contributes to many other stress and abrasion failures. The failure mechanism is called stress corrosion fatigue where a load is concentrated at a corrosion pit. Simply stated, corrosion on sucker rods is the reduction of a man-made material, steel, to its natural state or lower energy level. Elemental iron in steel combined with moisture or acid to forms other compounds, such as iron oxide, sulfide, carbonate, etc. Corrosion control is possible with an effective chemical inhibitor program. Chemical inhibition of sucker rods is accomplished by applying a film of inhibitor to the rod which acts as a barrier between the steel and its corrosive environment. Generally, well fluids will contain two or more corrodents that work together to cause attack on steel. Scales, such as iron oxide, calcium sulfate, iron sulfide, and iron carbonate, should be--but are often not--prevented from forming on sucker rods where they reduce the effectiveness of the chemical inhibitors.
When an adequate inhibitor film is not maintained on a sucker rod, the corrosion process accelerates, causing pitting of the rod. These pits represent weak points in the rod string because the cross-sectional area of the rod at the pit is reduced. Cyclic stresses, along with other induced stresses, cause the rod to commence cracking at these pits and eventually the rod will fail. Anytime a contaminated rod string is withdrawn from a well, wherein the rod surface does not have a visible coating of inhibitor, it would be desirable to clean the rod string down to bare metal and apply a film of inhibitor.
Accordingly, as rod is removed from the wellbore, it would be desirable to be able to present a clean inhibited rod at the wellhead which is much more satisfactorily handled by workmen as opposed to the contaminated rod. Moreover, it would be advantageous to clean and inhibit the rod as it is being withdrawn from the wellbore so that the threads thereof do not become contaminated with the foreign matter. Furthermore, it would be desirable for all sorts of various tubular goods, including the rod string, to be cleaned and inhibited so that while the tubular goods are stored, as well as when the string is replaced within the borehole, the life thereof is greatly extended.
Method and apparatus by which the above desirably features are attained is the subject of the present invention.